1. Field of the Invention
This invention relates to a combination of fluid extraction and membrane technologies in which unequal rates of transport through a nonporous membrane is achieved by solution and diffusion mechanisms driven by a difference in free energy of the preselected species across the membrane. More specifically this invention uses a supercritical fluid on at least one side of a nonporous membrane and at least one fluid that is fresh to the process on at least one side of the membrane after which the fluid is either recycled to the opposite side of the membrane or removed from the separation process. Fluids and membranes, including modified membranes, are provided to facilitate passage of one or more components of a mixture through the membrane while rejecting other components of the mixture.
2. Background
Reverse osmosis, as a commercial separation technique, has been available for over twenty-five years. In reverse osmosis, the osmotic pressure normally present between two solutions with different solute concentrations is reversed by using an externally applied pressure in the presence of a semi-permeable membrane. The membrane, placed between solutions of different concentrations, preferentially passes one component of the more concentrated solution into the more dilute solution, e.g., water from a saline solution to the pure water side. Much of the work in reverse osmosis has involved the development of new membrane materials and structures. Most membranes currently available are based on cellulose acetate, aromatic polyamide, and polyethyleneimine/polysulfone polymers. Variations in substituted polymer end groups are used to affect the flux and selectivity of a membrane for a specific application. The structure of the membrane is affected by the method of membrane deposition, the amount of cross-linkage, the temperature at which it is formed, and the speed at which the formed membrane is cured. Because of the number of chemical and structural variables available, membrane development has remained an empirical and costly art.
Because of the costly empirical nature of developing new membranes and the large number of membranes currently available, the current direction in membrane research is toward developing novel methods for the use of existing membranes. The approach is particularly evident in the field of biotechnology where conventional membranes are used in cell/protein and protein/protein separations.
Pervaporation is a membrane separation process where a liquid mixture is contacted with the membrane. A component of the mixture diffuses through the membrane and evaporates from the downstream surface of the membrane. Characteristically, a pervaporation system is operated by passing the liquid feed (at atmospheric pressure) over the surface of the membrane and removing the permeate vapor under vacuum on the downstream side of the membrane. The process is useful, in terms of effective separations at reasonable flux rates, only if the partial pressure of the permeating component(s) in the downstream gas phase is small relative to the upstream liquid vapor pressure.
The process suffers from two serious operational difficulties. First permeate vapor delivered at low pressure will have a low mass density and will require high volumetric fluxes in order to operate as an economical process. The high vapor flux requirements necessitate large trans-membrane pressure drop that increase capital and operating costs for a single-stage process to uneconomical levels. These costs can be reduced somewhat by staging the process. Second, the energy required to evaporate the permeate is not available from the sensible energy of the systems and must be supplied from external sources. Construction of the membrane module from thermally conductive materials, e.g., copper or aluminum, can provide the needed heat but makes the cost of the modules unacceptably high. The problem can be avoided by the addition of steam to the vapor side of the membrane. However, the steam temperature must be held below the boiling temperature of the upstream liquid mixture to minimize upstream vapor formation. Also, the stream-to-permeate ratio must be kept large enough to keep the partial pressure of the permeate in the stream low relative to the vapor pressure of the liquid mixture. This ensures the separation of the mixture components.
Perstraction is a membrane process whereby the permeate is removed from the downstream side of the membrane by a receiving liquid. The receiving fluid, if properly selected, has the effect of lowering the chemical potential on the downstream side of the membrane. Selection of the receiving liquid is critical to the concept. The receiving liquid must be 1) essentially incapable of permeation through the semi-permeable membrane, 2) lower in volatility than the permeating components, and 3) capable of dissolving a substantial quantity of the permeate. Examples of such materials include high-boiling, high molecular weight hydrocarbon oils, hydrotropes (e.g., sodium toluene sulfonate), concentrated aqueous solution of soap or micellizing surfactant, and concentrated, stable polymer lattices. It is noted that is unnecessary to recover all the permeate from the receiving liquid. It may be more economical to remove only a portion of the permeate from the receiving liquid and recycle the receiving liquid back to the membrane module. The ratio of receiving liquid to permeate can be as high as 20:1. The perstraction technique offers several advantages over other membrane separation processes. First, perstraction offers lower heat and power costs over competing process. Second, the permeation process can operate near atmospheric pressure without imposing significant stress on the membrane. Third, unlike pervaporation, the permeate is in a liquid rather than a vapor state; a fact which reduces the amount of downstream processing needed.
Although some effort has been made in using supercritical solvents with nonporous membranes (Schuker, U.S. Pat. No. 5,430,224), such efforts have focused on the use and recycling of the supercritical solvents on both sides of the membrane. Such recycling can result in the build-up of unwanted materials in the recycled solvent especially when either the permeate (components passing through the membrane) or the retenate (components not passed through the membrane) or both are not readily separated from the supercritical solvent. Further the energy used in separating and recycling the solvent often is not cost effective. And finally the energy costs associated with providing a supercritical solvent to both sides of the membrane are often not justified in many separation processes.
Accordingly, it is an object of the present invention to provide a membrane separation process that 1) extends the range of possible separations to a wider range of components, 2) improves the selectivity for passing or retaining various mixture components with respect to a nonporous membrane, and 3) renders the entire process more cost effective.
The process of the instant invention consists of first providing a nonporous (dense) membrane having two sides, a first side and a second side. The membrane may be in a flat sheet, spiral wound, a hollow fiber, or other suitably provided membrane configuration. Generally the hollow fiber configuration is preferred since it allows manifolding of fluids to the first side or second side or both sides of the nonporous membrane. The membrane is selected or otherwise treated to reject or pass various components from the mixture to be separated. For example, a membrane with hydrophobic properties or treated to have hydrophobic properties can be used to reject water and water-like mixture components from passing through the membrane while a membrane with hydrophilic properties or treated to have hydrophilic properties can be used to facilitate passage of water and water-like mixture components through the membrane.
The next step in the process is to provide a mixture for separation that has at least two components, a first component and a second component. Because of the wide variety of fluids and membranes combinable for a particular separation process, a wide variety of separations are contemplated by this invention including 1) pharmaceuticals and biological products in the field of biotechnology, 2) azeotropic mixtures such as ethanol/water and styrene/ethylbenzene, and 3) consumer products such as the removal of caffeine from coffee.
Next fluids are provided for the process with a first fluid being used on one side of the membrane and a second fluid being used on the second side of the membrane. Selection of appropriate fluids aids the separation process significantly. Thus a hydrophobic fluid is used on the second side of the membrane to enhance the rejection of a hydrophilic mixture component, that is, retard the passage of such a component through the membrane while the use of a hydrophilic fluid on the second side of the membrane facilitates and enhances the passage of a hydrophilic component through the membrane. In providing fluids, a key feature of this invention is that at least one of the fluids, that is, either the first fluid or the second fluid must be a fresh or virgin solvent with regard to the process. That is, the fluid on at least one side of the process cannot be recycled to that side of the process. Such use of fresh fluid has the advantage of avoiding a build-up of unwanted volatiles or hard to remove components that otherwise lessen the efficiency of the process. Although the fluid can not be recycled on the same side of at least one side of the process, the present invention does contemplate the use of the same fluid on both sides of the membrane, that is, recycle of the fluid from one side of the nonporous membrane to the other side of the membrane. In such situations, the fluid is essentially identical on both sides of the membrane. However, and as noted above, the use of different fluids on each side of the membrane is often desirable to facilitate and improve the efficiency of the membrane separation process. In any event, the fluid on at least one side of the membrane must be in the supercritical state.
Next the mixture to be separated is optionally combined with the first fluid to form a fluid mixture, it being noted that in certain instances the mixture to be separated can be passed by the membrane without an additional fluid. One of the components of the mixture must be being capable of passing through the membrane (i.e., the permeate) while the other component is retained or prevented from passing though the membrane (i.e., the retenate). new A driving force is then provided so that when the mixture is passed over the first side of the membrane, separation of the mixture occurs as a result of the first mixture component passing through the membrane to the second side of the membrane where it is removed by passing a fluid over the second side of the nonporous membrane. The second component of the mixture is retained on the first side of the membrane. The driving force is a concentration gradient although other gradients such as temperature and pressure may further promote the separation process.
The use of a fluid on either side of the membrane that retards or promotes the passage of various components through the membrane is an important aspect of this invention. For example, a fluid on the second side of the membrane (permeate side) can be chosen to facilitate the passage of one mixture component while retarding the passage of a second component of the mixture. Thus a hydrophobic fluid used on the second side of the membrane can be effectively used to retard the passage of a water or water-like component from the first side of the membrane. In sum, the use of one or more fluids or a membrane or both that facilitates and promotes passage or retention or both of selected components of the mixture through the membrane is a key feature of this invention. The permeate can be removed from the receiving fluid by flashing of the solvent or other removal techniques such as phase separation. Similarly the retenate can be removed from the first solvent by similar techniques.
The present fluid/membrane separation process minimizes the process energy required to separate the permeate from the extractant and improves the permeability and/or selectivity for the component to be separated over that of conventional separation systems. One way the present invention accomplishes this is through the feature of using a supercritical receiving fluid (permeate fluid) to remove the permeate from the downstream side of the membrane. Advantages to using a supercritical receiving fluid include: 1) no phase change of the receiving fluid affording a less energy-intensive separation; 2) increased selectivity of the membrane/fluid system; 3) increased permeability of the membrane/fluid system; and 4) the ability to effect difficult separations.
Removing the permeate from the backside of a membrane using a supercritical receiving fluid instead of a subcritical fluid offers a less energy-intensive method to recover the permeate from the receiving fluid. Conventionally, the permeate is recovered from the receiving fluid using distillation techniques which take advantage of the differences in vapor pressures between the two liquids. By using a supercritical receiving fluid, the permeate can be recovered by dropping the pressure of the mixture, thus causing a portion of the permeate to drop from solution. This technique takes advantage of the fact that the solubility of many permeates in supercritical fluids increases with increasing pressure. By dropping the pressure of the permeate/receiving solvent through an expansion valve, a portion of the permeate can be recovered. The amount of permeate recovered per cycle depends on the pressure drop through an expansion valve. When the solubility/pressure relationship for a specific system is inverse of that described, i.e., decreasing solubility with increasing pressure, the product can be recovered by reversing the expansion/compression operation.
The present technique offers the additional advantages of increased selectivity and increased permeability over conventional separation techniques because it takes advantage of the unique partitioning properties associated with supercritical receiving fluids. Exemplary supercritical receiving fluids include carbon dioxide, fluorocarbons, and low molecular weight alkanes such as ethane and propane. Other, more sophisticated supercritical fluids are available, but may be less preferred due to their toxicity, flammability or other undesirable properties. The receiving fluid preferably complements the membrane by having a partition coefficient favoring the permeate. This effect coincides with a lowering of the chemical potential on the back side of the membrane which increases the separation driving force across the membrane and increases the rate of permeate recovery.
The use of membrane and fluid characteristics facilitates and enhances mixture separation. The advantages of combining these parameters into a single process include reduced energy consumption, e.g., 85% energy savings over conventional distillation of ethanol/water mixtures to produce pure ethanol; the ability to complete product temperature sensitive separations at reduced temperatures; the ability to conduct in-situ separations in biologically-based processes to provide continuous extraction of product while improving reaction rates of the process by removing inhibiting products; and the ability to effect otherwise difficult separations.
As noted, the process of the instant invention features the use of a combination of membrane and fluid that are selected to pass certain feed component(s) through the membrane or reject other feed component(s) away from the membrane or both. In specific tests, supercritical carbon dioxide was selected as a solvent and three membranes (cellulose acetate, Teflon and TFC-185) were evaluated to separate a mixture of 86% water, 9% ethanol and 4.5% butanol. A cellulose acetate membrane showed no effect on the separation beyond what would have been expected from extraction with supercritical carbon dioxide alone. Teflon, a hydrophobic (water repelling) membrane, showed an enhancement in the alcohol content of the product stream. The TFC-185 hydrophilic (water seeking) membrane showed an increase in water in the product over that otherwise expected and an increase in alcohol content in the reject. In the production of pharmaceuticals and biological materials, separations are a critical factor in the cost of the product. In such applications, the process of this invention is used as a stand alone separation unit process or in-situ to the process producing these materials. Since biological products or by-products can poison or retard reaction rates in fermentors or reactors, the membrane separation technique of the present invention can be coupled with such reactors to remove product or by-products and enhance reaction rates. In addition, cleaned-up reject material from the feed side can be recycled to the process thereby saving chemical costs for a given process. Since the products in biological processes are often present in minute concentrations (one part in 100 or 1,000 or even 10,000) and are temperature sensitive, the present separation process offers the ability to complete these gross-cut separations (in a biological products sense) without thermally damaging the product thereby greatly reducing the cost of producing such products.